11th september 200711th september 2007 SESSION 2SESSION 2

MATERTIALS & STORAGEMATERTIALS & STORAGE

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Compatibility of materials with Compatibility of materials with hydrogen. Particular case: hydrogen. Particular case: Hydrogen assisted stress cracking of Hydrogen assisted stress cracking of titanium alloystitanium alloys

Azkarate I., Ezponda E. and Madina V.Azkarate I., Ezponda E. and Madina V.

INASMET-Tecnalia INASMET-Tecnalia Mikeletegi Pasealekua, 2 Mikeletegi Pasealekua, 2 20009 San Sebastian, Spain 20009 San Sebastian, Spain

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Hydrogen affects properties of materials causing a lost of ductility.Mechanical behaviour of a material can change from a Ductile to a Brittle one.

Rupture Ruptur

e

Ultimate Stress

Yield Stress

BRITTLE

DUCTILE

Strain

Stress

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Hydrogen embrittlement

HYDROGEN DAMAGE

External hydrogen

embrittlement

Internal hydrogen

embrittlement

Thermal gradientThermal contraction

Low-temperature embrittlement

Low-temperature effects

Hydrogen enters the material during

its processingMaterial

subjected to external hydrogen

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Hydrogen embrittlement

Hydrogen reaction

embrittlement

Interstitial solid solution

Hydrogen enters the material allong dislocations, grain boundaries, high

solubility phases…

Hydride embrittlement

Hydrogen attack

4H+C – CH4

Hydrogen recombination

H+H --- H2

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Hydrogen Assisted Stress Cracking of Titanium Alloys

Alloy N C H O Fe Al V Mo Ni Ti

Ti Gr-2 0,003 0,005 0,003 0,126 0,048 - - - - bal.

Ti Gr-5 0,011 0,020 0,008 0,144 0,18 6,4 3,8 - - bal.

Ti Gr-12 0,017 0,010 0,002 0,150 0,11 - - 0,26 0,66 bal.

Objective

The objective of the study has consisted of the evaluation of the sensitivity of titanium alloys to the Hydrogen Assissted Stress Cracking Phenomena.

Materials

The studied materials have been the titanium alloys Ti Gr-2, Ti Gr-5 and Ti Gr-12 (ASTM B-265) hot rolled and annealed.

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Experimental technique

The sensitivity of the studied materials to Hydrogen Assisted Stress Cracking (HASC) has been studied by means of the Slow Strain Rate Technique (SSRT).

The experiments were carried out using constant extension rate tensile testing machines of 50 kN capacity and selectable crosshead speed.

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Round tensile test specimens were located in Hastelloy C-276 autoclaves and attached to a fixed frame by one end and to the pull rod by the other using zircalloy fittings to ensure the electrical insulation of the specimens from the autoclave.

Specimens were tested in argon and in sea water at strain rates from 10-4 s-1 to 10-7 s-1, temperatures of 90 and 170 ºC, and an argon pressure of 10 bar.

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Hydrogen was cathodically produced on the specimen surface using a potentiostat. Specimens were located in the autoclave together with the reference (Ag/AgCl) and counter (platinum) electrodes, -1000 and -1500 mV (Ag/AgCl) being the applied potentials. Tests were performed at 90 ºC and a strain rate of 10-6 s-1. Load, position of the actuator, time and temperature data were continuously logged during the test.

After each test several mechanical parameters were measured to assess the loss of ductility and sensitivity of the studied materials to HASC.

Metallographic and fractographic studies by optical and scanning electron microscopy were also performed.

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Results

No loss of ductility for Ti Gr-2 and Ti Gr-5 alloys at any of the testing conditions, even in tests with polarization of -1000 mV. The same was noticed in the case of Ti Gr-12 when tested in argon and sea water. With -1000 mV polarization a slight loss of ductility manifested in the mechanical parameters of elongation and reduction of area was observed. With -1500 mV polarization an important loss of ductility and secondary cracks due to hydride precipitation were observed. The thickness of the titanium hydride layer formed on the specimen surface when tested with -1500 mV polarization, has resulted to be of up to 30 µm for the Ti Gr-2 alloy, 120 µm for the Ti Gr-12 alloy and has not been observed in the case of Ti Gr-5 alloy.

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Secondary cracking on lateral surface of Ti Gr-12 specimen tested at a strain rate of 2x10-7 s-1 in sea water with a cathodic polarization of -1500 mV (Ag/AgCl)

Hydride layer and secondary cracks on lateral surface of Ti Gr-12 specimen tested at a strain rate of 2x10-7 s-1 in sea water with a cathodic polarization of -1500 mV (Ag/AgCl)

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Comments

The different SCC behaviour of the studied titanium alloys when cathodic polarization is applied seems to be associated with the easy of penetration and solubility of hydrogen into the metal during slow straining. In order to explain this behaviour, the following aspects should be taken into account: The solubility of hydrogen in the alfa phase at the eutectic temperature (300ºC) is of 6,72% atomic (1500 ppm); this solubility is of 39% atomic (13200 ppm) at the same temperature in beta phase. The diffusivity of hydrogen in titanium alpha phase is considerably lower than in the beta phase. The β-phase content is negligible for Ti Gr-2 alloy, approximately 3% volume for Ti Gr-12 and 15% for Ti Gr-5.

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Back scattered electron image of Ti Gr-12 specimen tested at a strain rate of 2x10-7 s-1 in sea water with a cathodic polarization of -1500 mV (Ag/AgCl).

TiH2

TiH2

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Conclusions

No sensitivity to Stress Corrosion Cracking was observed for titanium alloys grades 2, 5 and 12 when tested in sea water at temperatures up to 170ºC. No sensitivity to Hydrogen Assisted Stress Cracking was observed for titanium alloys grades 2 and 5 when tested in sea water and with cathodic polarization of up to -1000 mv (Ag/AgCl). Ti Gr-12 alloy has suffered loss of ductility and secondary cracking due to hydride precipitation when cathodic polarization is applied to the specimen during the SSR test, that showing a clear sensitivity to Hydrogen Assisted Stress Cracking. The β-phase content plays an important role in the different behaviour of the studied alloys. The present work consists of an example of the hydrogen embrittlement of materials by the hydride formation type.

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